Declarative schema: Constraint with type="index" breaks setup process on subsequent upgrades

[k4emic]

Preconditions (*)

1. Magento 2.3

Steps to reproduce (*)

1. Use <constraint xsi:type="index"... in db_schema.xml
2. Run setup:upgrade twice

Sample db_schema.xml file

<?xml version="1.0"?>

<schema xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xsi:noNamespaceSchemaLocation="urn:magento:framework:Setup/Declaration/Schema/etc/schema.xsd">

    <table name="inventory_source_item">
        <column xsi:type="timestamp" name="updated_at" on_update="true" nullable="false" default="CURRENT_TIMESTAMP"
                comment="Update Time"/>
        <constraint xsi:type="index" referenceId="INVENTORY_SOURCE_ITEM_UPDATED_AT">
            <column name="updated_at"/>
        </constraint>
    </table>
</schema>

Expected result (*)

1. Setup process completes with no changes detected

Actual result (*)

1. Command fails with message PDOException: SQLSTATE[42000]: Syntax error or access violation: 1061 Duplicate key name 'INVENTORY_SOURCE_ITEM_UPDATED_AT'

[magento-engcom-team]

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@k4emic do you confirm that you was able to reproduce the issue on vanilla Magento instance following steps to reproduce?

• yes
• no

[magento-engcom-team]

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[k4emic]

Some speculation on why this happens:

The \Magento\Framework\Setup\Declaration\Schema\Diff\TableDiff::calculateDiff treats indexes and constraints very differently when the diff is calculated. The example column will be added correctly at first, but subsequent runs of setup:upgrade will identify it (correctly) as an index instead of a constraint, which leads Magento to believe the index should be created.

Changing the XML file to use <index ..> (and dropping the existing index) solves this issue.
I think this is mainly a documentation problem, since defining an index through the use of <constraint> is not really valid for two reasons:

1. It creates two (very similar) ways of doing the same thing
2. The less intuitive way doesn't work

I would propose dropping/deprecating support for <constraint type="index" in the XSD, while we are still in the early stages of declarative schema.
While this is in breach with the semantic versioning concept, the feature is already broken and likely misplaced.

[k4emic]

Doing a little more research, it seems like the <constraint> element is too loosely defined in what it should allow.

Something like <constraint xsi:type="table" name="my_table" /> and <constraint /> is totally valid according to the XSD.

<xs:complexType name="table">
        <xs:annotation>
            <xs:documentation>
                Table definition. Here we can found column, constraints and indexes
            </xs:documentation>
        </xs:annotation>
        <xs:choice minOccurs="0" maxOccurs="unbounded">
            <xs:element name="column" type="abstractColumnType"/>
            <xs:element name="constraint" />
            <xs:element name="index" type="index" />
        </xs:choice>

<constraint> is the only type with no type attribute of the three.
Something like the following could help developers choosing the right element type for their needs:

# vendor/magento/framework/Setup/Declaration/Schema/etc/constraints/constraint.xsd
<xs:complexType name="constraintType">
        <xs:attribute name="type">
            <xs:simpleType>
                <xs:restriction base="xs:string">
                    <xs:enumeration value="foreign" />
                    <xs:enumeration value="unique" />
                </xs:restriction>
            </xs:simpleType>
        </xs:attribute>
    </xs:complexType>

[magento-engcom-team]

✅ Confirmed by @engcom-backlog-nazar
Thank you for verifying the issue. Based on the provided information internal tickets MAGETWO-98318 were created

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